Tractor frames, to which the fifth wheels are mounted, typically comprise a pair of longitudinally extending C-shaped channel members that are supported by the wheel and axle assemblies. The channels are laterally spaced apart from each other and they provide a planar surface for resisting normal and twisting forces after the fifth wheel is secured thereupon and placed in operation, thereby transferring the payload of the trailer equally into each of the frame channels. In super heavy service applications, each of the channel members may even be comprised of double or stacked C-shaped channels. The fifth wheel can be mounted to the truck frame in a number of various ways, but for the sake of this discussion, only the fifth wheels which indirectly mount to the channels through use of drop-over angle brackets or flat mounting plates will be discussed.
The drop-over method utilizes an L-shaped angle member which conforms and communicates with the top and vertical legs of each of the channels, having its vertical leg bolted to the outboard vertical side of the channel. The horizontal leg of the drop-over bracket, which coexists with the top horizontal leg of the channel, also includes an arcuately shaped mounting bracket which is welded thereupon. The arcuate mounting bracket lies generally above the top horizontal leg of the C-shaped channel and the exact location is a function of the variation of the width of the channel horizontal leg and the lateral spacing between the channels of the tractor frame, as will become clearer later in the discussion. The arcuate mounting bracket includes a horizontally aligned throughbore for accepting a coupling crosspin which is in communication with the fifth wheel framework. More specifically, the fifth wheel framework has a flange on one or each peripheral edge that communicates with the arcuate mounting bracket. In this way, flange ports on each flange can be horizontally aligned with the mounting bracket throughbore for pinning the fifth wheel to the drop over angle mounting bracket and hence, to the tractor frame. Although the drop-over brackets transmit lateral inertial forces which are caused by curving and rocking of the fifth wheel into the frame, drop-over arrangements are best suited for resisting longitudinal forces and moments caused from acceleration, deceleration, as well as the downward forces acting on the ramp's eccentric, to the rear of the bracket pivot center during coupling. In any event, the lateral and longitudinal forces will produce high magnitude lifting forces between the bracket and the tractor frame and as a result, localized forces have developed fatigue cracks either directly beneath the arcuate mounting bracket near the weld line, or between the bolt holes.
Another type of anchoring assembly is the flat plate mounting assembly, and it offers increased fatigue cracking strength compared to the drop-over bracket method during the lateral and lateral overturn loads, while being as effective in resisting vertical loads, The flat mounting plate assembly consists of a rectangular, planar plate of steel which is secured flatly on top of each of the tractor channels, with the lateral sides of the plate extending slightly beyond each channel. In order to transfer loads from the plate into the frame, a section of L-shaped angle is used to support the plate directly below the overhang. Those in the art typically refer to this angle as the outboard angle member. The outboard angle member is simultaneously secured to the tractor channel and the flat mounting plate by bolting its horizontal leg to the bottom side of the mounting plate and then bolting its vertical leg to the vertical wall of the channel. The same arcuate mounting brackets used in the drop-over mounting assembly are welded to the top of the mounting plate and they are generally located above the top horizontal leg of the C-shaped channel: their relation to the frame again being a function of the channel width and the lateral spacing between the track channel members. The location of the arcuate mounting bracket, with respect to the horizontal leg of the channel member, does not depend upon which assembly is used.
The flat mounting plate method of attachment offers increased fatigue strength when compared to the drop-over mounting assembly because the lateral forces and moments which work upon the arcuate mounting bracket are first distributed throughout the entire flat plate surface area before they are distributed and transferred into the outboard angle member. This arrangement offers greater surface area for load distribution, thereby decreasing the magnitude of stresses being transferred into each attachment bolt and into the critical areas between the anchoring bolts. However, one disadvantage of the fiat mounting plate arrangement is that it utilizes a large, thick steel plate which adds cost and weight to the mounting arrangement, as well as extra time and labor for assembling it to the tractor frame.